1. Field of the Invention
The present invention relates to thermal printers wherein the selective energization of heating elements causes the transfer of dye to a receiver member.
2. Description of the Prior Art
In a thermal printer which uses a dye transfer process, a carrier containing dye is disposed between a receiver, such as paper, and a print head assembly formed of, for example, a plurality of individual thermal elements often referred to as heating elements. The receiver and carrier are generally moved relative to the print head which is fixed. When a particular heating element is energized, it is heated and causes dye to transfer (e.g. by sublimation) from the carrier to an image pixel in the receiver. The density, or darkness, of the printed dye is a function of the temperature of the heating element and the time the carrier is heated. In other words the energy delivered from the heating element to the carrier causes dye to transfer to an image pixel of a receiver. The amount of dye is directly related to the amount of heat transferred to the carrier.
Thermal dye transfer printers offer the advantage of true "continuous tone" dye density transfer. This result is obtained by varying the energy applied by each heating element to the carrier, yielding a variable dye density image pixel in the receiver.
A conventional method of energizing heating elements employs a pulse width modulation scheme as will now be explained. A print head is organized into a plurality of groups of heating elements. The heating elements in each group are simultaneously addressed in parallel. In this disclosure when the term addressed is used, it means that an element is capable of being energized. Each group is addressed sequentially one at a time. The reason groups are used is that if all the heating elements were energized at the same time, a large and more expensive power supply would be needed. For example, if a heating element were to draw 68 millamperes and 512 heating elements were used, the power supply would, if all heating elements were energized, have to produce 33.3 amperes. A very expensive power supply would have to be provided to produce this amount of current. Therefore, the group arrangement is preferred.
When a group of heating elements are addressed during an address cycle, individual elements of the group can be selectively energized. The heating elements, when energized, are driven with a constant current. FIG. 1a shows a prior art pulse width modulation scheme used to drive a heating element. As noted above, the amount of dye transferred to an image pixel of a receiver depends upon the energy (heat) transferred to the carrier. Since the receiver moves relative to the print head image pixels are longer than their corresponding heating elements. During the address cycle, the maximum time a current pulse can be provided to a heating element is (t.sub.1 -t.sub.0). This will produce the maximum density dye image pixel. Image pixel #1 in FIG. 1b is formed by this process. If the pulse width is made smaller (t.sub.b -t.sub.0), then a less dense image pixel will be formed. See image pixed #2 in FIG. 1b. Thermal pixels are selectively turned off at the variable time t.sub.b depending on the desired dye density of the image pixel. If a still smaller pulse width (t.sub.a -t.sub.0) is used then an even lower dye density image pixel will be formed. See image pixel #3 in FIG. 1b. During the time dye image pixels are being formed, both the dye carrier and receiver are moved relative to the heating element print head. The reason for this is that a stationary carrier can stick or bond to the heated heating elements. If a thermal pixel is energized for a time less than (t.sub.a -t.sub.0), then on an image pixel such as shown image pixel #4 of FIG. 1b will be formed. For illustrative purposes, the size of a heating element (stippled portion) which produces these image pixels is shown next to image pixel #4.
FIG. 1b shows the first line of four image pixels formed by the above described system. The area of the dye portion of an image pixel is proportional to the image pixel dye density. At the beginning of an address cycle (t.sub.o FIG. 1a) all the heating elements of a group are generally energized. The energization of each heating element of a group starts at the time t.sub.o (START OF LINE). At this time t.sub.o, the heating elements of a group create a peak current demand. Thus, even with using groups of heating elements, problems still remain with drawing too much current at the start of an address cycle.
Although the image pixels of FIG. 1b are shown as having dye portions (stippled) and undyed portions (white), the undyed portions may have some dye due to the thermal time constant of the heating elements. In other words, after a heating element has been de-energized it may still provide enough heat to cause some dye to transfer to an image pixel.